The Truman Doctrine was a foreign policy perspective that stressed that the United States must help other countries to contain and prevent the spread of Communism. Truman said in a 1947 speech: “I believe it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures.” His comments referred to U.S. aid to Greece and Turkey to counter the threat of Soviet influence.

The Truman Doctrine helped lead to active U.S. involvement in the North Atlantic Treaty Organization—a military agreement between various European countries known by its acronym NATO.